Perovskite related ceramic oxides, comprising, for example, alkaline earth metal-copper oxide, such as orthorhombic yttrium-barium-copper oxide materials, usually characterized as YBa.sub.2 Cu.sub.3 O.sub.7-x or "1:2:3 ceramic oxides", are well-known "high temperature" superconductor materials. This 1:2:3 ceramic oxide material has been found to provide electrical superconductivity, i.e., essentially no electrical resistance, in the region of 93.degree. K.
The 1:2:3 ceramic oxides and other alkaline earth metal-copper oxide based ceramics can operate in the superconducting mode well above the 77.degree. K. boiling point of relatively inexpensive and plentiful liquid nitrogen, and could find increased use in composite windings for high current magnets, energy storage coils, long distance power transmission, and the like. However, 1:2:3 ceramic oxides and other superconducting ceramic oxides are hard and brittle, and by themselves are not easily extruded or otherwise fabricated into fine wire or windings. Additional problems stem from "weak links" formed, for example, by grain boundary contaminant films and non-conducting interparticle films.
As a solution to the brittleness problem, Jin et al. in Applied Physics Letters, "High T.sub.c Superconductors-Composite Wire Fabrication", Vol. 51, No. 3, Jul. 20, 1987, pp. 203 to 204, placed a metal cladding around a heat treated 1:2:3 ceramic oxide powder superconducting core. The metal cladding, Ag, or Cu with a Ni/Au oxygen diffusion barrier, allowed ease of drawing into fine wire form, from 0.6 cm to 0.02 cm diameter, and also provided a parallel electrical conduction path in case the ceramic oxide lost its superconducting properties and became a normal conductor. Ag was found particularly advantageous as a cladding, since it could have the dual function of cladding and oxygen diffusion medium. The drawn wires were then annealed at 900.degree. C. and 600.degree. C. in oxygen. Multi-filamentary ribbon composites were also formed. Jin et al. also recognized the problem of oxygen loss from the metal clad 1:2:3 ceramic oxide during sintering, suggesting addition of an oxygen donor to the core, use of a perforated or porous cladding, or the like.
Early efforts to improve ductility while maintaining the high strength of the sheathing for a superconducting core, were made in U.S. Pat. No. 4,863,804 (Whitlow et al.), where Nb.sub.3 Sn, Nb.sub.3 Al or the like were placed in composite sheathing, and a copper or aluminum inner layer was dispersion hardened with from 0.01 wt. % to 1 wt. % of Al.sub.2 O.sub.3, ZrO.sub.2, SiO.sub.2, TiO, Y.sub.2 O.sub.3, Cr.sub.2 O.sub.3, Th.sub.2 O.sub.3, SiC or BC. Alloys, such as Cu-Nb, Cu-Ta, or Al-Fe were also taught as effective. The highly ductile outer layer could be substantially pure, oxygen free, electrical grade, high conductivity (OFHC) copper, or electrical conductivity grade aluminum. Such copper or aluminum outer sheaths would be 99.95% pure minimum, and 99.75% pure minimum, respectively (for example, copper alloy 102, and aluminum alloy 1060 or 1175; from Standards Handbook, "Copper.Brass.Bronze-Wrought Mill Products", Copper Development Association, p. 11, and Aluminum Standards and Data 1979, The Aluminum Association, p. 15, respectively).
In U.S. Pat. No. 4,971,944 (Charles et al.), an attempt was made to prevent oxygen loss during sintering, yet still allow close interparticle contact, by electroless deposition of Au from a solution of gold chloride, and organic solvent on superconducting oxide.
What is needed however, is a superconducting composite that would optimize properties of ductility, interparticle contact, and minimum oxygen loss, and provide a safeguard in case of reversion to normal (non-superconducting) activity. It is the main object of this invention to provide such a composite.